It is generally known that defects, e.g. in the form of cracks, have a negative effect on the strength of a component and thus also the reliability of the component. Under certain conditions, such cracks in the surface of components may grow. This applies, in particular, in the case of components such as, e.g. shafts, turbine blades or, in particular, turbine blade roots which, apart from static loading, also experience dynamic loading.
Methods for determining defects in components are represented in DE 44 21 277 A1, DE 37 31 947 A1, WO 85/02464 A1 and EP 0 337 700 A2. In the aforementioned documents, the dye penetration method used for crack testing a surface is described in each case. In these methods, a contrast medium is applied to the surface to be tested for cracks, which penetrates into existing cracks. The contrast medium is removed, the surface is cleaned, if necessary, and a developer is applied with a thin layer thickness. The contrast medium which has penetrated into the cracks becomes mixed with a developer at the geometric locations of the cracks so that the crack contour is rendered visible due to the thin layer of the developer. For this purpose, the developer, for example, has a white color and the contrast medium has a red color or the contrast medium is fluorescent.
Using the aforementioned method, however, it is not possible to investigate the roots of turbine blades in the installed state.
A further method for determining defects in components is represented in the book “Werkstoffprüfung mit Ultraschall” (Material testing with Ultrasonics), J. & H. Krautkrämer, Springer-Verlag Berlin 1986. In particular, it is described on pages 110 to 111 how a component area can be examined by means of ultrasound with a transmit probe and a receive probe by utilizing the reflection of an internal area in the component. When outsides of the component are opposite one another, the probes are arranged opposite one another on these outsides. The transmit probe sends the ultrasonic signal to the area to be examined. From there it is reflected to the receive probe. A further possibility of examination is the tandem method in which both probes are arranged behind one another on the same outside. In this arrangement, a component backside is utilized for a second reflection. However, none of the methods described can be used if there is only one accessible outside of the component and, in addition, there is no suitable backside for the additional reflection according to the tandem method.
A further method for determining defects in components is the so-called eddy current test method. In this method, an alternating magnetic field which produces eddy currents in the material to be tested is generated by means of an exciter coil. The eddy currents, in turn, result in an alternating magnetic field which is measured by means of a detector coil. Material faults in the form of defects have a characteristic influence on the magnetic field measured and can thus be detected.
Stress corrosion cracks or defects can lead to progressive damage in the case of dynamic loading of rotating parts such as, e.g. a wheel disc on a rotor of a turbine. It is necessary, therefore, to check such components for defects and stress corrosion cracks in predetermined intervals. For economic reasons, wheel discs and turbine blade roots or turbine blades which are arranged secured against twisting by axial or radial pins on a rotor cannot be tested by taking the rotor apart or dismantling the turbine blades. It is advisable, therefore, to perform ultrasonic testing in the area of the shrink fits and the adjoining material areas, particularly in the area of the inner hub surface and the axial torsion locks in the case of wheel discs. However, the geometry of the wheel disc is extremely complicated for an ultrasonic test with ultrasonic probes in the inaccessible inner hub area and in the vicinity of torsion locks. Moreover, the reproducible performance of ultrasonic tests also encounters other problems. The only available coupling surface for the ultrasonic probes are the side cheeks of the wheel disc. The accessibility to the coupling surfaces on the wheel disc is frequently restricted by adjacent wheel discs. A further problem consists in that the sound paths to geometry-related reflectors and to any cracks which may be present and emanate from the hub hole can be identical.